A kneading extruder 501 as shown in FIG. 5 has conventionally been known, for instance, as a kneading extruder that kneads a plurality of types of raw materials to produce kneaded material and that extrudes the kneaded material (see Patent Document 1, for instance).
As shown in FIG. 5, the conventional kneading extruder 501 includes a screw shaft 511 or screw shafts 511 having a single-shaft structure or double-shaft structure, a barrel 512 in which the screw shaft(s) is rotatably placed in an inside space thereof, a raw material feeder 513 for supplying the raw materials into a space between the barrel 512 and the screw shaft(s) 511, a die 516 that is in communication with a barrel extrusion port 514 which is an outlet of the barrel 512 for the kneaded material, the die 516 having a die extrusion port 515 through which the kneaded material supplied from the barrel 512 is extruded to outside of the device, and a booster pump 517 that is provided between the barrel 512 and the die 516 and that supplies the kneaded material, supplied from the barrel extrusion port 514, to the die 516 while boosting pressure of the kneaded material, for instance.
A hopper 518 is connected to a raw material supply port 522 provided on the barrel 512, and the raw materials supplied from the raw material feeder 513 are supplied through the hopper 518 into the barrel 512. The raw material feeder 513 has a drive motor 513a of which number of rotations can be controlled, and a desired quantity of the raw materials are supplied from the raw material feeder 513 through the hopper 518 into the barrel 512 by control over the number of rotations of the drive motor 513a. 
The screw shaft(s) 511 (single shaft or two shafts) placed in the barrel 512 is connected to a screw driving device 519. The screw driving device 519 has a drive motor 519a of which number of rotations can be controlled, and the screw shaft(s) 511 is driven to rotate with a desired number of rotations through control over the number of rotations of the drive motor 519a. 
A gear pump is used as the booster pump 517, for instance, and is driven at a constant speed so that a constant quantity of the kneaded material can be supplied to the die 516 while pressure thereof is boosted. A mesh-like filter 520 is provided between the booster pump 517 and the die 516 thereby preventing foreign matter or the like into the die 516.
In the conventional kneading extruder 501 having such a configuration, the pressure of the kneaded material at the barrel extrusion port 514 is detected by a pressure detection unit 521, and the number of rotations of the drive motor 513a of the raw material feeder 513 and the number of rotations of the drive motor 519a of the screw driving device 519 are controlled on basis of the detected pressure of the kneaded material so that the pressure of the kneaded material at the barrel extrusion port 514 is controlled so as to be kept constant. Such control is generally referred to as feedback control method.
On condition that the pressure higher than a preset pressure is detected at the barrel extrusion port 514 by the pressure detection unit 521, specifically, the number of rotations of the drive motor 513a of the raw material feeder 513 is decreased so that the quantity of the raw materials that are supplied from the raw material feeder 513 into the barrel 512 is decreased. Concomitantly, the number of rotations of the drive motor 519a of the screw driving device 519 is decreased so that quantity of the kneaded material that is extruded from the barrel extrusion port 514 of the barrel 512 is decreased. Thus the pressure of the kneaded material at the barrel extrusion port 514 can be decreased.
On condition that the pressure lower than the preset pressure is detected at the barrel extrusion port 514 by the pressure detection unit 521, the quantity of the kneaded material that is extruded from the barrel extrusion port 514 is increased by increase in the quantity of the raw materials that are supplied from the raw material feeder 513 into the barrel 512 and increase in the quantity of rotations of the screw driving device 519. Thus the pressure of the kneaded material at the barrel extrusion port 514 can be increased.
In the conventional kneading extruder 501, in this manner, the quantity of the kneaded material that is supplied from the barrel extrusion port 514 through the booster pump 517 to the die 516 can be kept constant by the feedback control for keeping the pressure of the kneaded material at the barrel extrusion port 514 constant.
The conventional kneading extruder 501 including the screw shaft 511 having a single shaft structure, for instance, is used for applications for secondary processing in which the kneaded material made from compounded pellets as raw materials is extruded so as to mold tubes, films or the like. In recent years, the device including the screw shafts 511 having double shaft structure can be used for applications in which tubes, films or the like are directly molded.
In the conventional kneading extruder 501 shown in FIG. 5, however, the control method is employed in which variable control over the extrusion quantity (conveyance quantity) of the kneaded material in the barrel 512 is performed by variable control over the supplying quantity of the raw materials from the raw material feeder 513 and variable control over the number of rotations of the screw shafts 511 so that the pressure of the kneaded material at the barrel extrusion port 514 is kept constant.
Change in the number of rotations of the screw shafts 511 causes change in shear rate for the kneaded material, which exerts an influence on kneading property of the kneaded material. In addition, change in residence time of the kneaded material in the barrel 512 causes change in thermal history of the kneaded material, which also exerts an influence on the kneading property of the kneaded material. On condition that two or more types of the raw materials are mixed, change in the supplying quantities of the raw materials exerts an influence on mixture ratio of the kneaded material, in particular, the greater the number of shafts forming the screw shafts is, the more conspicuous the influence becomes.
There is a time lag (time difference on the order of several tens of seconds, for instance) until the raw materials supplied into the barrel 512 are extruded as the kneaded material from the barrel extrusion port 514. It is therefore difficult to perform follow-up on the pressure of the kneaded material at the barrel extrusion port 514 that is changing every moment, in the method in which the supplying quantity of the raw materials from the raw material feeder 513 is controlled on basis of the pressure of the kneaded material at the barrel extrusion port 514. It is more difficult to perform the follow-up on change in the pressure on condition that a plurality of types of raw materials are supplied at different supply locations into the barrel 512. Variability in the number of rotations of the screw shafts 511 in addition to variation in the supplying quantity of the raw materials causes change in the rate of screw shear that is important among conditions of kneading and thus makes it impossible to avoid change in physical property of the kneaded material.
In the conventional kneading extruder 501, accordingly, the physical property of the extruded material extruded from the die 516 is unstable and there is a fear that unstability in molded state may exert an influence on quality of products, depending upon combination of the supplied raw materials, form of the end product and/or the like. In particular, there is a fear that such an influence may arise more conspicuously on condition that the molding is carried out with use of the kneading extruder in small size having a comparatively small extrusion quantity. In the kneading extruder in which films, tubes, or the like are directly molded with use of extruded material therefrom, state of molded bodies is unstable because the kneading property of the extruded material directly influences conditions of the molded bodies. On condition that two or more types of the raw materials having different specific gravities are mixed, it is still difficult to stabilize the kneading property.
An object of the invention is to solve above problems and to provide a kneading extruder for kneading a plurality of types of material to produce kneaded material in melted state and extruding the produced kneaded material by rotation of screw shafts which device is capable of compatibly achieving extrusion of the kneaded material in a constant quantity and stabilization of kneading property.